An image signal obtained from an image pickup system configured by an image pickup element, an associated analog circuit, an A/D converter and the like contains a noise component in general. This noise component can be roughly divided into a random noise and an impulsive noise. The random noise is generated in the image pickup element and the analog circuit and has a characteristic close to a white noise characteristic. On the other hand, the impulsive noise is a noise mainly caused by the image pickup element and represented by a defective pixel or the like.
With regard to random and impulsive noise reduction processing, for example, JP2007-110445A discloses a technique in which an image signal is photographed several times, the result is recorded in a plurality of frame memories, and the maximum value and the minimum value are removed and averaged or a median is taken in relation to the image signal on the same coordinate. As a result, a random noise and an extrinsic impulsive noise, which is not fixed as a defective pixel, can be removed, and a high-quality image signal can be obtained.
Also, as shown in JP2005-318126A, an example in which a random noise amount and an impulsive noise amount are estimated and noise reduction is performed in single reduction processing common to the both noises is disclosed. As a result, the random noise and the impulsive noise can be removed, and a high-quality image signal can be obtained. Also, since noise reduction is performed in single reduction processing, noise reduction processing with less occurrence of discontinuity or artifact caused by the noise reduction processing can be realized.
Moreover, with regard to the reduction processing of a defective pixel, for example, JP2003-69901A, discloses a technique in which after correction or generation processing is performed in relation to a defective pixel measured in advance, a plurality of noise reduction processing results such as a low-pass filter or a median filter are weighted and added. As a result, occurrence of discontinuity or artifact caused by correction or generation processing of a defective pixel can be suppressed, and a high-quality image signal can be obtained.